I've narrowed this down to having Netflow enabled and capturing local on both master/backup. As soon as you enable it on the backup, the HAproxy health checks on the master immediately fail. Disable Netflow on the backup, and they immediately start working again.

Steps to reproduce:

1. Run two instances of OPNsense.2. Configure high availability (I'm using CARP and XMLRPC sync, but I am not using states sync).3. Configure HAproxy on the primary. Observe that health checks show your backends/frontends as UP.4. Enable Netflow on master.5. Observe that HAproxy health checks still work.6. Enable Netflow on backup.7. Observe that HAproxy health checks now show your backends/frontends as DOWN.